Full frame would have to be the first choice, for me. I think these days, it's a lot closer than it used to be though and crop sensors have improved. Expect more noise in shadows with a crop sensor and it's a little more important to get a good exposure, as full frame is a bit more forgiving. There's a bit more to it though, light, iso, dynamic range and so on.
I use a 5Div and an R7, the R7 is noisier than the 5D but that's not to say I haven't had noisy files out of my 5D if I've bombed the exposure.
Not forgetting, noise reduction software has come on leaps and bounds too and noisy files can be very easily improved.
Also, depending on what you want to take pictures of, a crop sensor will effectively increase the focal length of your lenses compared to full frame, for example, a 100mm lens on a Canon crop sensor would be 160mm, whereas it would be 100mm on a full frame Canon camera.
This is a potentially a big subject but in a nutshell, I'd go full frame for better image quality but that said, crop sensors have caught up from what it used to be, you might be hard pushed to tell any real difference with the newer cameras. My R7, often staggers me with what it can do.
Bear in mind, sensor size is one thing, you need good glass to get the best out of it.